In recent years, the development of LSI technology has been significant, and the size of LSIs is becoming even-smaller. Thus, the width of wiring through which signals are transmitted is also getting smaller, i.e., wirings are becoming ultrathin. As the resistance values of conventional Tungsten (W) films and Aluminium (Al) films are unable to withstand this thinning of the wire, the adoption of copper (Cu), which has a low resistance value, has been suggested.
However, the adoption of copper film, with its lower resistivity causes several problems that do not occur in the case of adopting WorAlfilm. One problem is that copper diffuses to the circumference of the LSI, and when this happens the function of the LSI is largely lost.
Therefore, a method was proposed for forming an alloy film for preventing copper diffusion (a barrier metal: TiN, TaW, ZrN, VN, TiSiN, etc.) as an underlayer film on a substrate (for example, see “The latest developments in copper wiring technology”, Realize Corporation, May 30th, 1998, pp. 3–171).
There is, however, a lower limit on the thickness of such alloy film that allows the alloy film to efficiently function as a barrier metal. If the thickness is below the lower limit, the alloy film does not function as a barrier metal. On the other hand, the width of copper wiring is predicted to fall below 0.1 μm in the future. Therefore, if the thickness of the barrier metal cannot be further reduced, the thickness of the copper wiring film alone will have to be reduced, and when this is coupled with the prediction that the width of the copper wiring will be under 0.1 μm in the future, the cross-sectional area of the copper wiring will become too small, which makes the adoption of copper with low resistivity meaningless. In other words, the limitations of alloy films such as mentioned above are becoming apparent.
Another problem is that the adhesion of conventional barrier metals to copper is inferior. For example, a copper film (a plated copper film or a CVD copper film) readily peels off from either a Si or SiO2 underlayer film. In particular, the adhesion characteristics between a copper film and an underlayer film are reported to be worst in the case where a Tantalum(Ta)-based film, which is considered to be the most efficient of the above-mentioned barrier metals is arranged as the underlayer film. In this case, the copper film may even peel off as it forms on the Ta-based film.
Therefore, it is an object of the present invention to solve the problems such as those mentioned above which exist in conventional barrier metals, and to provide an underlayer film-forming material for copper, a method for forming an underlayer film for copper, an underlayer film for copper, and a semiconductor device, which enable the prevention of copper diffusion even if the thickness of underlayer film is thinner than that of conventional barrier metals, and which enable superior adhesion to copper wiring film.